The present invention relates to a head for the extrusion of two thermoplastic layers to be deposited for example upon a steel pipe. Heads of the type to which the invention pertains are used for example to deposit a coating of a thermoplastic adhesive such as an ethylene copolymer upon a steel pipe and simultaneously a jacket of a thermoplastic material such as polyethylene is extruded on top of the thermoplastic adhesive.
The German printed patent application No. 1957078 discloses an extrusion head which includes a first annular housing member, an intermediate ring and a second annular housing member whereby the first housing member together with the intermediate member establishes an annular opening and nozzle for the depositing of the first inner layer such as the adhesive, and the intermediate ring together with the second annular housing member establishes a second gap and a second nozzle opening for depositing the outer synthetic envelope. The first housing annulus, the intermediate ring and the second housing annulus are arranged essentially in axial direction one behind the other, but the nozzle openings i.e. the ends of the two annular gaps terminate in a common plane.
This particular printed patent application provides annular ducts for feeding the adhesive and the thermoplastic material whereby baffle rings are disposed in these annular channels. The feeder duct and annular channel for the adhesive is particularly arranged in the first mentioned housing annulus; the intermediate ring includes a heating structure and the second annular housing member includes the feeder duct and annular channel for feeding the thermoplastic material which becomes the outer envelope. The two annular feeding ducts and the baffle ring respectively arranged therein are disposed one behind the other but in the same plane and they have similar diameters. The baffle rings are adjustably disposed using steel screws which are disposed outside the extruder head to shift the baffle rings in relation to their longitudinal axis, i.e. excentrically, in order to obtain a desired adjusting position. These baffle rings and particularly their inner circumferential surfaces extend into the annular ducts and are of an elliptical configuration. The purpose thereof is that the adhesive as well as the synthetic for the outer coating can be distributed in a uniform layer thickness with reference to the circumference of the steel pipe but under consideration of the differences in viscosity.
Experience has shown that in some cases an adjustment of the baffle rings during operation has to be carried out against a relatively high working pressure of the adhesive and of the liquidous plastic, the pressure being for example 100 to 300 bars at an operating temperature from 150 to 170 degrees Centigrade. It is a distinct disadvantage that the adjustment has to be carried out here with a very high force. The requisite force has frequently resulted in a permanent deformation of the baffle rings and that of course leads to an uneven layering and coating thickness. Moreover, the baffle rings had to be exchanged, in fact quite frequently.
In accordance with a different aspect it has been found that the annular ducts as described in the above mentioned German patent application exhibit orientations of the gaps for the adhesive and the outer coating material which are angled off, the annular feed ducts run to some extent perpendicularly toward the steel pipe to be jacketed. Consequently, the travel path for the material is relatively long, particularly through the extruder head and toward the common exit and nozzle opening resulting in an increase in counterpressure at rather high operating temperature. Since the adhesive is often sensitive to high temperatures, it is conceivable that the material decomposes to some extent resulting in a deterioration of the adhesiveness particularly which affects directly the bonding of the jacket which is to be made to adhere to the steel pipe by that adhesive. Any deterioration in the adhesion may result in corrosion inside the extrusion head because the decomposing produces a certain amount of acid.